The present invention relates to a receiving system for digital data which are transmitted by means of the sixteen-state amplitude and phase modulation method commonly referred to as 16 QAM and relates more specifically to a carrier recovery arrangement for such a system.
The 16 QAM modulation is obtained by adding together two quadrature carriers, each being modulated in amplitude (levels 1 or 3) and phase (0 or .pi.). The modulated signal can be represented as: EQU x(t)=(2a.sub.k +1) sin (.omega.t+b.sub.k .pi.)+(2c.sub.k +1) cos (.omega.t+d.sub.k .pi.)
(where a.sub.k, b.sub.k, c.sub.k, d.sub.k are each equal 0 or to 1 and t is located between kT and (k+1)T, T being the duration of a symbol). The article "Design and Performance of a 200 Mbit/s 16 QAM Digital Radio System", by I. Horikawa, T. Murase and Y. Saito, published in IEEE Transactions on Communications, December 1979, Vol. COM-27, No. 12, pages 1953-1958, describes a comparatively simple carrier recovery arrangement for 16 QAM modulation. As shown in FIG. 1, which represents the signal point configuration for 16 QAM modulation, only eight of the sixteen signal states of this modulation method have phases equal to .+-..pi./4 or .+-.3.pi./4, that is to say the same phases as the four-state phase-shift-keying method known as 4 PSK. The authors of the article therefore recommend to perform the carrier recovery operation only then when the signal has one of these eight states, and so to recover the carrier of a 16 QAM signal by means of a carrier recovery loop for a 4 PSK signal.
The disadvantage of this prior art method becomes rapidly apparent. If it is assumed that all states of the 16 QAM modulation have equal probabilities, the recovery loop would, statistically, operate only during half the time. But if the transmitted signal contains a long sequence whose states have phases differing from .+-..pi./4 and from .+-.3.pi./4, the loop is not active during this sequence and the loop runs the risk of unlocking, i.e. no longer tracking the phase.